Adhesion between the glass and the interlayer is the most critical, controllable parameter related to the penetration resistance of laminated safety glass. If the adhesion is too high, the laminate fails as a monolithic unit upon impact, and hence does not offer the occupant of a motor vehicle much protection. If the adhesion is too low, sharp pieces would separate from the laminate on impact, and could thus cause injury to the occupant.
Laminated safety glass today are mainly made from float glass. For example, the automobile windshield is made from two pieces of float glass which have been bent either by heat sagging or by heating followed by form-pressing. The bent glass pieces are bonded together by a plasticized polyvinylbutyral (PVB) interlayer. In the manufacture of float glass, the glass is cast onto and transported on top of a bath of molten tin. While one of the surfaces is in contact with tin (the tin-side), the other is usually in contact with an inert atmosphere such as nitrogen (the air-side). Consequently, the chemistry of the two surfaces of the same glass sheet can be quite different. Sometimes, the difference in surface chemistry manifests itself in the adhesion between the glass and the PVB interlayer. One gets higher or lower adhesion depending on whether the xe2x80x9ctinxe2x80x9d side or the xe2x80x9cairxe2x80x9d side of the glass is in contact with the PVB interlayer, among other factors such as PVB moisture, inherent adhesivity of the interlayer, bulk glass chemistry. The difference in adhesion, often referred to as asymmetric adhesion, can be so much that a laminate is deemed usable as a windshield in one glass orientation, but not in another. Some laminators are frustrated with having to identify and keep track of the glass orientation, or having to install extra equipment to flip the glass to achieve a certain prescribed orientation.
It is therefore an object of this invention to provide a laminar structure which is usable as windshields and side-glass in automobiles such that the difference in adhesion between the PVB interlayer and the tin-side and that between the PVB and the air-side of the glass is reduced. This invention is also applicable to other glass/adhesive sheet laminar structures in which asymmetric adhesion is experienced.
In accordance with this invention there is provided a glass/adhesive sheet laminar structure comprising at least two layers of glass and a sheet of plasticized polyvinylbutyral (PVB), said polyvinylbutyral having blended therein an ionizable metal salt as an adhesion control additive to provide a preselected level of adhesion between said layers of glass and said sheet of polyvinylbutyral which is suitable for use as automobile windshields and side windows and body glass, and incorporating another ionizable metal salt which provides a cation different from that provided by the adhesion control additive such that the content of the cation which is provided by the leveling agent is between 0.03 and 1.35, and preferably 0.07 to 1.1 milliequivalents/kg (meq/kg) of sheeting. The second metal salt may be a salt of an alkaline earth metal such as magnesium or calcium or a transition metal such as zinc or copper, or Group IV metal such as tin. Surprisingly, this small amount of salt or leveling agent is adequate in reducing, and in some cases, essentially eliminating asymmetric adhesion without other deleterious effects on the overall performance on the laminate such as haze. Moreover, the adhesion level of the PVB interlayers may be controlled by conventional means by adjusting the amount of adhesion control additive.